Nanopatterning boosts efficiency of incandescent bulbs

PORTLAND, Ore.  Using ultrafast lasers to inscribe nanoscale patterns on the filaments of standard incandescent lightbulbs, researchers at the University of Rochester (N.Y.) were able to increase the bulbs' light-emitting efficiency dramatically, letting a 60-watt bulb shine as brightly as one rated for 100 W.

The nanoscale patterns make the tungsten lightbulb filament a "black body" that is much more efficient at radiating light, the researchers said.

"We patterned the tungsten filament of a lightbulb by shining a femtosecond laser on it right through the glass. As a result, [the filament] glowed more brightly, but without changing the amount of energy it uses," said Rochester professor Chunlei Guo. "In a production environment, our pulsed laser could be used to treat the filament material before it is inserted into the glass bulb."

Guo performed the work with research assistant Anatoliy Vorobyev.

The optics researchers have been experimenting with using lasers to inscribe nanoscale patterns into the surface of metals to change their color without having to use filters that cut down on reflected light. The team had learned how to change the surface characteristics of metals with a femtosecond pulsed laser to reflect such colors as blue, gold, grey and black. Since an ideal black body radiates all wavelengths equally, the researchers decided to try it on a tungsten lightbulb filament. The result was the observed boost in efficiency.

Now the team is working to tailor the color of incandescent bulb filaments, not only to boost their efficiency but also to enable their light emission spectrum to be customized for different applications.

Femtosecond lasers provide super-intense beamsas much as intensity as the entire U.S. power grid concentrated onto a spot the size of a needle's point, but only for a brief slice of time, equivalent to a single second in a span of 32 million years.

The brief burst of intense patterned heat from the femtosecond laser at the surface of the metal vaporizes elements of its structure so that they settle into a predictable nanoscale pattern that traps certain wavelengths of reflected light. When an incandescent bulb's filament is so patterned, then turned on, it radiates at the frequencies at which reflected light would have been trapped by the surface. The blackened filament radiates more intensely at more frequencies than an untreated filament and is thus more efficient.

Guo and Vorobyev now are working to control the size and shape of the inscribed nanostructures in a bid to characterize processes that could impart finer control over the spectrum of colored light reflected and radiating from the metal surface. They have not yet been able to produce pure colors, such as solid blue, but have succeeded in tailoring the spectrum of emissions, such as adding blue to make incandescent bulbs radiate cooler temperatures of light. The team has also been successful in creating partially polarized light by inscribing tight, parallel rows of nanostructures on the filament surface.